Method of refining zinc ore



Patented Sept. 30, 1952 f Ulti-TED` ASTM.lz-:s`

l t t y lf l Y METHOD OREFINIGHNCGRE- v vvR'VYI-HQ'1li' F-"Orrland George '1.'Junl'{in',l Fort' f j Smith,"Ark., assignors to The Residue Company,

, Fort Smith, Ark., a corporation of Arkansas .Abplicatibn Novenibplo, 1948 serial No. 59396 `2 claims. (C1. vs-v) The present'inventio'n. relates to a method for the recovery of`zinc fromazinc-concentrate.`

, InV general, theproduction of vzinc froma'zinc concentrate may be accomplished by roasting,

`followed by sintering andfurnace reduction, or ,A

'Percent Grade Zinc vHigh Grade 90. 5-99. s

Intermediate... 1 99. 35 Brass Special.. 98.7 lfrime Western. 98. 0

A The standard specificationsio'r slab zinc rf `th 'e various grades is indicated below.:

t .Percent (Max.)

Grades l l Sum of Pb Fe 'Cd Pb; se,

. .f Cd

High Grade.; 0. 010 0. 020. 0.070 .0,100 Intermediate.. 0.200 0.030 0.500 0.500 Brass SpecialA 0.600 '0. 030 0. 500 1:000 Prime Westernmf. .Y.... 1.600 0.080

The electrolytic method, lwhile in generali-producing the highest grade zinc, has the disadvantage of relatively high operating cost; I y y Y In all methods Yof zinc production one-ofthe prime considerations is the conversion of the zinc sulde present in'zinc concentrate to the oxide, since the presence of the sulfide is not desirable, and is even harmful, Ain the `final distillation step for the ,recovery of` the substantially pure zinc. Consequently, it has been the generalpracticeto reduce the "sulphur content of the ore 'or concentrate-to between about 3% and 7% by roasting. followedbysintering, which further reducesthe sulphur 'content'to ya minimum prior to the distillation step.' `VV y In the well known Waelz process, zinc residues l lpare zinc metal 'in 'a quality heretofore unobtain- Vand other zinc-bearing materials are. treated with vapors as well asI the carbon monoxide.

'which-arevolati'liaed in the process. lAn oxidizing atmosphere, maintained above the charge in the kiln, then oxidizes the escaping metallic The fumes and gas from' thev kiln are treated in a bag house or by Cottrell precipitatorsto recover a 'predominantly zinc oxide product which may be subsequently distilled in a conventional retort furnace to produce metallic zinc. 3

,In ,another process, sufficient sulphuris re-A -moved by roasting a zinc sulfide ore to bring the roasted fore to asulphur content of 4% or less, vthe 'resulting zinc oxideV is then treated with a carbonaceous reducing agent` together withja chl-oridizing agent such as sodiumy chloride for the eliminationoi" cadmium, lead and sulphur, and the resulting sintered cake, alleged to; be a substantially pure zincv oxide cake, is subsequently distilled in a furnace.

IAn object of the present invention is to preable` in the normal retort smelting process. Another objectof ythe present invention is to uprepare a high quality zinc Without thenecessity of adding solid carbonaceous reducing agents or chloridizingagentsprior to the distillationy stage.

Another object 'of theinvention is'to provide a process for the manufacture of high quality zincV providing for better Zinc furnace recovery and lower cost in the smelting operation.

Another object of the present invention is to provide a'process for they production of high quality metallic' zinc While utilizing ra relatively high sulphur content zinc concentrate.

Still another `object of theinvention is to pro- I vide a method for treatingfzinc concentrates whereby the normal impurities such as cadmium and lead may be recovered as valuable byproducts.

,cined zinc concentrate, nodulizing vthe resulting .zinc concentrate in a rotary kiln, and reducing thel zinc oxide nodules in a conventional retort furnace.

y By reducing` a` high sulphur content concentrate, normally containing rbetween 28% and 30% sulphur to a sulphur percentage of 5 to l0,- as -compared with the range of 4between about 3% to 7%. as in normal practice, z'aciirisi'derable sav.-

ing in roasting vcos'tfis effected. We havefound that when a calci-ned zinc'r concentratecontainporosity, normally about 15%. K

kporosity is determined by measuring the .actual 3 ing from about to about 10% sulphur is nodulized las in a rotary kiln, the nodules produced have a `high apparent density, yet also a high porosity, and may be treated in a` conventional furnace to give a zinc metal of a quality heretofore `unobtainable bythe normal retort smelting process- .Where the preliminary roasting is carriedout to vyield a calcined zinc concentrate containing less than 5 sulphur, an amount of high sulphur content greenconcentrate may Abe added tobring the sulphur content into the Y desired range.

The yfeed to the nodulizing-kiln may be Wet or dry and may havepractically anyv particle size. More specifically, the calcined zinc concentrate from the pre-roasting process is passed through a ranging in maximum dimension from a fraction of inch to several inches. The nodules, containing a high percentage of zinc oxide, leave the rotary kkiln at its lowermost portion and are then ypreferably cooled. The nodules in heated condition are extremely tough, but by 'cooling the nodules, they are mademore susceptible t'o'crushing,` to obtain properly sized material for introduction, after mixing with the reducing agent, into the retort furnace.

`Inr Ythe rotary kiln, substantially all of the cadmium and sulphur, and most of the lead, are volatilized, but only a very minor proportion of the zinc present-,is volatilized. The 'volatilized metals'are recovered in collection apparatus such as bag houses, Cottrell precipita-tors, cyclone separatoraland the like. The treatment inthe rotary nodulizing kiln eliminates substantially all ofthe sulphur content ofthe original charge as sulphur dioxide.

Normally, only a small amount, .as low as 1%, of the zinc originally present in the charge' is volatilized in the nodulizing furnace, but even this small amount `may be recovered in the precipitation apparatus.

The nodules leaving vthe rotary kiln are characterized by relatively ihigh apparent density, on .the order of from about 137 to about 189 pounds per cubic foot, and an apparently uniform The percentage density and the' apparent density, subtracting the latter from the former, and calculating the percentage of the actual density representedv by ther difference. The high density permits increased retort or other type zinc furnace capacity with Yconsequent lower cost in the smelting operation. The high porosity of the particles permits better zinc furnace recovery due to the intimate contactl of the zinc WithY the reduction agents within thefurnace, whether solid or gaseous reducing agents are used.

Reaction conditions Within the nodulizing kiln may be varied over a considerable range depend- Y ing on the percentage of sulphur in the calcined vzinc concentrate used las the feed material. In general, the temperature Within the kiln is on the order of about 2,000 to 2,300y Whenthe l sulphur content of the calcined zinc concentrate is in the range from about 5% An excess of air is essential in the operation of the rotary'nodulizing kiln to maintain an essentially oxidizing atmosphere Within the kiln.

Reduction ofthe amount ofairin the kiln causes increased lvolatiliz'ation of zinc, whi`ch is highly undesirable. The proper amount of air, as Well as the proper temperature, to be used may be determined by observation of the kiln operation. The air V'supply and temperature are adjusted so that any substantial burning of zinc, evidenced by a green name Within the kiln, is avoided.

lfr neasure of mechanical maintenance being re- The nodules leaving the rotary kiln are cooled and crushed, roughly to 1/4 to each in size, for admixture Witha reducing agent in the retort furnace, as hereinbefore mentioned.

The resulting crushed nodules may then be reducedy in a conventional retort furnace by means of the customary reducinghagents.v

We have found that the v-slab zinc resulting rfrom'a reduction of the nodulized material re sultsin a higher quality product that has vbeen previously obtained innormal retort operation.

For example, a typical analysis of a slabzinc prepared from nodulized material by our vprocess shows0.014% iron, 0.05%'lead and a traceof cadmium, leaving a zinc content of approximately 99.9%. This product'is Well Within the speciiications for Vpremium y quality High Grade zinc. The majorr proportion of the zinc produced by this process would classify either as interme diate or as High Grade, Whereas zinc produced by the normal sintering and fur'nacingprocesses yields essentially Prime Western and Brass Special grade zinc, when using normally available zinc concentrates. v y

It can be seen that the invention provides a process for the production of high quality zinc form high porosity, making it particularly amenable for reduction in a retort furnace. The process itself permits continuous uninterrupted operation of the nodulizing step, with only a small quired. There is no substantial Vgeneration of noxious fumes and an almost total absence of dusting, thusv providing excellent` working conditions for the operatin'gpersonnel. A'

A specic example of the process4v ofi the l,invention will be described vwith referenceft'o ythe attached flow diagram. )A green zinc concentrate composed 'largely of zinc sulphide and analyzing 60% zinc, ,32,% sulphur, 0.3% cadminumand 1.07% leadjs charged into a roasting kiln Wheeinthe green zinc con- ,centrate is partially oxidized. The calcined zinc concentrate producedv lcontains approximately 65%Mzinc, 8% sulphur,-0.33%.cadmium and 1.2% lead. This calcined concentrateis next -fed to the upper end of aninclinedrotary nodulizing furnace which is maintained under Asubsttmtially oxidizing conditions. The fumes from the nodulizing kiln are collected in appropriate fumecollectors for the recovery `of cadmiumand lead.

The resulting nodules leave the lower fend 'of;the

inclined kiln and contain approximately 71% Zinc', V0.05% to 0.08% lead, and traces of lsulphur' to about 10%.

and cadmium. The nodules have an apparent density of about 137 to 189 pounds per cubic foot.

The nodules, preferably cooled and ground to a size of approximately 1A inch, and then passed to a conventional zinc smelting furnace to be treated with appropriate reducing agents. The slab zinc recovered from this furnace contains about 0.05% to 0.10% lead and only a slight trace of cadmium.

Many modifications may be made in the process herein describedwithout departing from the spirit of the invention, and it is not our intention to limit the scope of the invention other- Wise than necessitated by the scope of the appended claims.

We claim as our invention:

1. The process for the recovery of zinc from a sulphur-containing zinc ore, which comprises simultaneously tumbling and heating a zinc ore product containing from about 5% to 10% of sulphur at a sintering temperature of from 2000 to 2300 F. in the absence of solid carbonaceous material and in the presence of an amount of oxygen in excess of that stoichiometrically required to oxidize the sulphur present to sulphur dioxide, continuing the tumbling and heating until substantially all of the sulphur, cadmium, and most of the lead have been volatilized and recovering a lsintered product containing zinc oxide and not over about 0.08% of lead and only a trace of cadmium, said product being in the form of nodules of substantially uniform porosity that are particularly amenable after being ground for reduction in a retort furnace to re` cover high quality metallic zinc therefrom.

2. The process for the recovery of zinc from a calcined zinc concentrate produced by the roasting of a high sulphur-containing zinc ore, which comprises subjecting a mass of such calcined zinc concentrate containing initially from about 5 to 10% by weight of sulphur to a rotating and tumbling action, simultaneously with such action heating said calcined zinc concentrate to a sintering temperature of from 2000 to 2300 F. in the absence of solid carbonaceous material and in the presence of an amount of oxygen in excess of that stoichiometrically required to oxidize the sulphur present to sulphur dioxide, continuing the rotating, tumbling and heating until substantially al1 of the sulphur, cadmium, and most of the lead have been volatilized and recovering a sintered product containing zinc oxide and not over about 0.08% of lead and only a trace of cadmium, said product being in the form of nodules of substantially uniform porosity that are particularly amenable after being ground for reduction in a retort furnace to recover `high quality metallic zinc therefrom.

RAYMOND F. ORR. GEORGE T. JUNKIN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS OTHER REFERENCES Handbook of Non-Ferrous Metallurgy, by Liddell, v01. 1, published by McGraw-Hill Co., Inc., 1945, page 303. 

1. THE PROCESS FOR THE RECOVERY OF ZINC FROM A SULPHUR-CONTAINING ZONC ORE, WHICH COMPRISES SIMULATNEOUSLY TUMBLING AND HEATING A ZINC ORE PRODUCT CONTAINING FROM ABOUT 5% TO 10% OF SULPHUR AT A SINTERING TEMPERATURE OF FROM 2000 TO 2300* F. IN THE ABSENCE OF SOLID CARBONACEOUS MATERIAL AND IN THE PRESENCE OF AN AMOUNT OF OXYGEN IN EXCESS OF THAT STOICHIOMETRICALLY REQUIRED TO OXIDIZE THE SULPHUR PRESENT TO SULPHUR DIOXIDE, CONTINUING THE TUMBLING AND HEATING UNTIL SUBSTANTIALLY ALL OF THE SULPHUR, CADMIUM, AND MOST OF THE LEAD HAVE BEEN VOLATILIZED AND RECOVERING A SINTERED PRODUCT CONTAINING ZINC OXIDE AND NOT OVER ABOUT 0.08% OF LEAD AND ONLY A TRACE OF CADMIUM, SAID PRODUCT BEING IN THE FORM OF NODULES OF SUBSTANTIALLY UNIFORM POROSITY THAT ARE PARTICULARLY AMENABLE AFTER BEING GROUND FOR REDUCTION IN A RETORT FURNCE TO RECOVER HIGH QUALITY METALLIC ZINC THEREFROM. 